Disorder-order transition in mesoscopic silica thin films

Electron microscopy has been used to study the mesoscopic (nanometer-level) and microscopic (micrometer-level) structural evolution of mesoscopic silica thin films grown at the air-water interface under dilute, acidic (pH < 2) conditions. Transmission electron microscope observations reveal that the film begins with a disordered (amorphous) structure. Over time, mesoscopically ordered regions (hexagonally packed cylindrical channels) nucleate and grow within the film. Scanning electron microscopy reveals microscopic structural features such as ribbons, protrusions, domain boundaries, microindentations, and pits. Our work shows that mesoscopic order develops within the film through a "disorder to order transition." Our observations also clarify the role of the air-water interface in confining film growth to two dimensions during the initial stages. We note that a two-dimensional (in-plane) to three-dimensional (unconstrained) growth transition occurs when the film exceeds a critical thickness. We extend the current understanding of the structural evolution of the film by providing a detailed mechanism for the development of mesoscopic order and microscopic features and consider the possibility of a universal growth mechanism for films and particles.